chore(deps): update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -222,7 +222,7 @@ jobs:
       run:
         working-directory: examples/proto
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: bazel-contrib/setup-bazel@0.15.0
         with:
           # Move autoload_externally to .bazelrc once Bazel 6 support is dropped.